In drainage pipeline systems within buildings, plastic tubes are widely used, in particular made of PVC-U (rigid polyvinyl chloride), PP (polypropylene) and PE (polyethylene). In these systems, referring to the metric dimensions, the outer tube diameters employed rarely exceed 200 mm, whilst the smallest diameter used is normally 32 mm The commercial lengths of the tubes are short relative to the tubes used in sewers or in pressurised fluid distribution pipelines, because the structure of the building is poorly suited to the installation of tubes longer than 3 metres. Commonly employed lengths, normally available on the market, are: 150 mm, 250 mm, 500 mm, 750 mm, 1000 mm, 1500 mm, 2000 mm and 3000 mm. The most widely used technique for joining tubes is the bellmouth with a sealing elastomeric gasket: the end of the tube is widened and provided with a seat for a gasket, in order to enable the insertion of another tube into the end, achieving a junction with fluid-dynamic tightness.
The end of the tube that is inserted into the bellmouth is chamfered, so insertion into the bellmouth is facilitated and the risks of damaging the gasket are reduced. The nominal commercial length of the tube with bellmouth does not consider the bellmouth portion, because the length of the bellmouth is irrelevant in calculating the extension of the pipeline. Tubes with bellmouths at both ends also have considerable commercial success.
Plastic tube production lines are extrusion lines with continuous production in which the extruded tube advances along the line at uniform velocity (extrusion velocity). In the line is normally present an automatic cutting machine, commanded by an electronic control unit, able to obtain tube segments with chamfered end. The length of the cut segments corresponds to the nominal commercial length plus a segment with sufficient length to obtain, with subsequent bellmouth making machine, with thermoforming process, the bellmouth. In draining tubes within buildings, the products most on demand are short tubes, usually tubes having commercial length of up to 500 mm.
The traditional automatic cutting machine is configured as a carriage that moves within a frame along the axis of the tube. Within the carriage is located a drum comprising two rings, separated by spacers, within which is obtained a cavity that is coaxial to the tube. In the drum is located the cutting tool. The drum is able to rotate at high velocity around the tube. Since the tube is in constant rectilinear motion, when the cut is performed the carriage must also move at the same velocity as the tube. At the time the cut is executed, two clamps positioned on the carriage at the side of the cutting assembly, close on the tube, achieving a rigid carriage-tube structure that moves at the same velocity, thereby allowing maximum cutting precision. The electronic control unit receives the signal that commands the execution of the cutting cycles from an electronic position measurer that, through an electro-mechanical transducer (wheel-encoder), constantly measures the velocity of the tube and the required lengths of tube to be cut.
When the cut is commanded, the carriage starts from a motionless condition and from a starting position, follows and reaches the point to be cut, synchronises to the velocity of the tube, closes the clamps and through the cutting tool performs the cutting cycle. Once the cut is completed, the clamps release the tube and the carriage returns to the starting position, awaiting another cutting command. It is evident that the higher the extrusion velocity, the greater will be the length of travel needed by the carriage to complete the work cycle. It is also evident that, for equal extrusion velocity, the shorter the lengths of tube required, the greater will be the number of cuts the machine must execute in a unit of time. To limit the length of the working stroke of the carriage and increase, for equal stroke available, the number of producible short segments, the so-called “flying” cut technique described in the patent EP 0129515 is advantageous. This technique enables to achieve working cycles characterised by sequences of short segments alternating to a long segment. With the “flying cut” control technique, the carriage provided with a shearing cutter does not take as a reference the absolute stroke-start position, but the relative position on the tube where the next cut is to be executed with respect to the instantaneous position of the carriage. By so doing, after the first cut, the carriage in the return stroke does not return to a stroke-start position, but when it arrives in the vicinity of the position of the tube where the next cut is to be executed, it stops “on the fly”, it reverses its motion and it reaches the cutting point, it synchronises the velocity with the extrusion velocity and it carries out the cutting cycle, and so on until the end of the working stroke. After completing the working stroke, the carriage returns to the stroke-start position and from said position it can cut a long segment and then resume the sequence of cuts “on the fly” that produce the short segments.
The technical evolution of the extrusion lines is characterised by a constant increase of the extrusion velocity, whilst the application of drainage tubes in buildings requires prevalently short segments with bellmouths. To meet this requirement, no relevant problems are associated with installing at the end of the extrusion line multiple bellmouth-making machines, able to sustain the arrival in a given time of an ever greater number of tubes to be shaped with bellmouth ends. However, it is necessary to increase ever more the velocity with which the tube segments are produced in order to keep pace with the velocity of extrusion of the tubes.